@Veevivianvee , I can totally relate to all of your emotions and panic. My sweetie has horrible PTSD from childhood abuse and periodically shuts down for up to two weeks. This last time, she sent me a heart emoji every couple days, which eased my panic a bit.
What has really eased my panic, however, are the following:
(1) Reading these boards and as many books I can get my hands on about PTSD, CPTSD, and trauma. I've learned to understand the disorder as a chronic illness and sort of imagine my sweetie has Cystic Fibrosis and has to carry around an oxygen tank all the time and periodically her lungs fill up with fluid and she needs to tend to her own survival. I had to stop thinking her behavior was related to me or our relationship.
(2) Address my own issues: When she isolates, that's when I read the the trauma/ptsd books and look deeply into my own reactions. From this introspection, I have learned that my panic is from my own life, my own CPTSD, my own childhood. I have attachment trauma which manifests as an anxious attachment style. My sweetie is more avoidant (for good reason!). Two people with PTSD, however, doesn't have to be the kiss of relationship death. With self-awareness and patience, you can being a loving, forgiving presence to each other, and help each other heal. But you really do have to throw expectations about how relationships are supposed to be out the window. Don't compare to anyone else.
Hang in there.